1. Field of the Invention
The present invention relates to an image forming apparatus, such as an electrophotographic printer that uses a liquid developer. More particularly, the present invention relates to a developer controlling apparatus for a developing roller that uniformly controls an amount of developer deposited on a developing roller, an image forming device having the same, and a developer controlling method thereof.
2. Description of the Related Art
Generally, an image forming device, such as an electrophotographic printer, forms an electrostatic latent image on a photoconductor, such as a photoconductive belt or an organic photoconductive drum (OPC). The latent image is developed with a developer having a predetermined color. The developed image is transferred onto an image receiving medium, such as a sheet of record paper (P), thereby obtaining a desired image.
Such an electrophotographic image forming device is classified into a wet type or a dry type, depending on the developer employed therein. A wet type electrophotographic image forming apparatus uses a liquid developer formed by mixing powdered toner with a liquid carrier having volatile components.
FIG. 1 shows a conventional wet type electrophotographic color printer 1 using a liquid developer.
As shown in FIG. 1, the wet type electrophotographic color printer 1 includes an image forming unit 5.
The image forming unit 5 includes four image forming units, for example K, C, M and Y image forming units, to form an image having four colors, that is, black (K), cyan(C), magenta (M), and yellow (Y).
Each of K, C, M and Y image forming units is provided with a photoconductor 9 such as an OPC drum. An electrification roller 12 is disposed adjacent to the photoconductor 9 for electrifying the surface of the photoconductor 9 with a predetermined electric potential. A laser scanning unit 11 emits a light beam onto the electrified surface of the photoconductor 9 to form an electrostatic latent image having a low electric potential thereon.
Below the photoconductor 9, a developing device 13 is disposed. The developing device 13 develops the electrostatic latent image with liquid developer 48 having a predetermined color, that is, K, C, M or Y. The liquid developer 48 also has a density ranging from about 3% through 20% solid. Consequently, a developer image 49 (see FIG. 2) is formed having a density in the range of about 20% through 25% solid.
As shown in FIG. 2, the developing device 13 includes a storage part 6, a developing roller 7, a deposit roller 14, a metering roller 15, and a cleaning roller 16.
The storage part 6 reserves a liquid developer 48. The developing roller 7 is located below the photoconductor 9. The deposit roller 14 is located below the developing roller 7. The deposit roller 14 and the developing roller 7 apply predetermined electrical forces to the liquid developer 48 to form a difference in electric potential ΔV, that is, a deposit vector V, therebetween. Due to the difference in electric potential ΔV, the liquid developer 48 is deposited on the developing roller 7, thereby forming a layer of developer thereon. The layer of developer has a high density in the range of 12% through 20% solid and a uniform amount of developer M/A. The metering roller 15 is located on an upper portion of the developing roller 7 and substantially over the deposit roller 14. The metering roller 15 applies a predetermined pressure to the layer of developer formed on the developing roller 7. At the same time, the metering roller 15 applies a predetermined electric force to the regulated layer of developer to ensure that it remains on the developing roller 7 and does not attach to the metering roller 15.
When the layer of developer formed on the developing roller 7 moves to a nip between the developing roller 7 and the photoconductor 9, and comes to contact with the photoconductor 9, a predetermined difference in electric potential is formed between the developing roller 7 and the electrostatic latent image having the low electric potential formed on the photoconductor 9. The layer of developer is attached to the electrostatic latent image of the photoconductor 9, by the predetermined difference in electric potential, so that the electrostatic latent image of the photoconductor 9 is developed into a developer image.
The cleaning roller 16 is located on the opposite side of the lower portion of the developing roller 7 from the deposit roller 14. The cleaning roller 16 cleans developer remaining on the developing roller 7 after the electrostatic latent image of the photoconductor 9 is developed.
To uniformly control an amount of developer M/A deposited on the developing roller 7 by the deposit roller 14, the conventional printer 1 further includes an amount-of-developer controlling unit 70. The amount-of-developer controlling unit 70 controls deposit vector V by determining an applied voltage for the developing roller 7 and/or the deposit roller 14 on the basis of conductivity of the liquid developer 48.
The amount-of-developer controlling unit 70 includes a sensor part 71, a memory part 77, and a control part 74.
The sensor part 71 has a conductivity sensor to sense conductivity of the liquid developer 48. The conductivity sensor is disposed in the storage part 6 of each of the developing devices 13 of the K, C, M and Y image forming units 5, and submerged under the liquid developer 48.
The memory part 77 stores a plurality of values of deposit vector V predetermined by experiments. The plurality of values of deposit vector V are determined as values which can obtain a target amount of developer M/A according to varying conductivities.
The control part 74 selects a corresponding value of deposit vector V among the plurality of predetermined values of deposit vector V stored in the memory part 77 according to the conductivity sensed by the sensor part 71. The control part 74 controls a voltage applied to the developing roller 7 and/or the deposit roller 14 according to the selected value of deposit vector V.
However, as shown in FIG. 5, the conductivity of the liquid developer 48 generally has a characteristic that varies according to density (% solid), and also an electric charge Q/M (coulomb per mass) for the same density.
Further, as shown in FIG. 6, an amount of developer M/A deposited on the developing roller 7 has a characteristic that varies according to the conductivity of the liquid developer 48, and also the electric charge Q/M of the liquid developer 48 for the same conductivity.
Accordingly, if the value of deposit vector V is determined only by the conductivity, the amount of developer M/A which is actually deposited on the developing roller 7 may be different from the target amount of developer M/A as the density and/or the electric charge Q/M of the liquid developer 48 varies. Therefore, in this case, the layer of developer may not form on the developing roller 7 uniformly. As a result, the quality of final image, such as image density, image uniformity, dot reappearance ability, line reappearance ability, and a color gamete, can deteriorate.
As another method of controlling the deposit vector V during developing, an amount-of-developer controlling apparatus (not shown) has been proposed to control the values of deposit vector V on the basis of density.
However, like the amount-of-developer controlling unit 70 for controlling the values of deposit vector V on the basis of the conductivity of the liquid developer 48, since the amount-of-developer controlling apparatus determines the values of deposit vector V only with the density of the liquid developer 48, it also presents a problem in that the amount of developer M/A actually deposited on the developing roller 7 may be different from the target amount of developer M/A as the density and/or the electric charge Q/M of the liquid developer 48 varies. Thus, a layer of developer may not form uniformly on the developing roller 7.
Accordingly, to correctly and precisely deposit the amount of developer M/A on the developing roller 7 during the developing, and thereby uniformly form the layer of developer on the developing roller 7, it requires that the deposit vector V base determinations on the consideration of all factors including conductivity, density and an electrical charges, which can affect the amount of developer M/A, rather than one factor such a conductivity or a density.
Accordingly, there is a need for an improved image forming device including a developer controlling apparatus that controls voltage applied to a developing roller and/or a deposit roller based on a variety of factors.